Can I replace HU without affecting any controls?

It really depends on the brand/model of the unit you buy. I would say that most units (particularly from good brand like the ones you mentioned) will support it. I was looking at the Pioneer AVIC series, and you need an adapter for most (all?) vehicles.

I'm not sure about such things as the touch tracer display, however...

 

I would find another stereo shop cause unless you think the stock speakers are really terrible they are trying to sell you more than you need. I just install a Alpine W505 in my prius II without replacing the speakers and it sounds much much better. I do plan on changing the speakers also but later when I have more time. I do have Steering wheel controls but no source change, just volume up/down and track up/down. You will need to add a PAC adapter for the steering wheel functions to work.

The head unit cost me $799.00 and the JVC camera cost $120. I needed the Nav and Bluetooth unit so it came out to some $1300. But then again I did all the labor myself. You will need to find a face plate trim to fit the standard Double DIN unit which only available thru a UK Toyota Dealer. PM me if you need links.

 

See, I would tell you it's pretty much of a "system", yes you will get some benefit of upgrading the head unit, but if you don't do the speakers and probably amps as well, it won't be as effective.

 

I believe Frenchie has played both with Kenwood and Pioneer...

Keep in mind, you must purchase the HU plastic adapter - the only one around is the one in a later thread. That adapts the opening to 200mmx100mm (standard Toyota size). Someone has also identified an additional adapter (in the thread) to take the 200mm opening down to 180mm (standard Kenwood et al). It looks like any adapter (steering) that works with Toyota and works with Prius (I bought a RAV4 HU with steering built in - it works fine).

Plastic adapters alone will run you around $140+ delivered.

There's nothing that says you must upgrade speakers right away. The HU install is at most a .5 hour job - and simple (with the right adapter cables). Do the HU, and then see how you like the speakers. n My speakers sound much better with a new HU...

 

I just finished the install of my Alpine W505. With all the wiring including the backup camera, it took me about 2.5 hours. As it stands the PAC SWI-JACK works with the exception of the source button. The alpine units seems to be much quicker than my Kenwood 8120 to boot and interface is faster but not as nice and clean. I installed the nav unit also but am holding off the Bluetooth unit as I am missing a cable.

I agree with rrolff that the system does sound much better even with the stock speakers.